Eddy Current Sensor

Eddy current sensor detect defects or distances to metal objects dynamically without contact, with precision, They are commonly employed to measure ferromagnetic and non-ferromagnetic substances. They are acceptable for applications in harsh industrial environments due to their exceptional tolerances, such as oil, dust, moisture and field interference. Flexible and miniature versions offered, which can also be used for distance measurements at that location.

Eddy Current Sensor

What is the principle of eddy current?

Electric currents are induced in metals by changing the vortex current magnetic field. Eddy current flows in a closed circular path within the conductor, vertical in the plane magnetic field.

Are eddy currents AC or DC?

The current is always directed unilaterally in the case of DC, where as it is alternately in the case of AC. In DC, there is no oscillation whatsoever. However, eddy currents, having no definite direction, are moving in nature. The concepts of AC and DC do not apply to these currents, as there is no specific direction of flow of these currents.

Currents are not sent back to the source, as they are electromagnetically induced. So generally the path between the source coil and the load, which in this case, is the conductor surface, is incomplete. Streams do not return to the source. The conductive surface, where the eddy has actually been produced, dissipates their energy in the form of heat. And since the circuit is technically open, it can be said to be neither AC or DC.

What is the difference between eddy current and induced current?

An eddy current is an induced current, but in metals or materials where you do not want induced current such as the core or mounting frame of a transformer.

How is eddy current created in a conductor?

When a conductor is placed in a time-varying magnetic flux, the change in flux due to the varying magnetic fields induces small loops in the conductor, and current flows through these loops via Faraday’s law. These currents are known as eddy currents.

Eddy Current Sensor Operating Principle

Vortex current sensors use the principle of eddy current formation to determine the displacement. It is formed when a changing magnetic field cuts a conductor. Relative motion causes a circulating current of electrons, or electrons, in the conductor. These circulating eddies of electromagnets with magnetic fields that oppose the effect of applied magnetic fields. The more powerful the magnetic field, or the higher the conductivity of the conductor, or the greater the relative speed of motion, the larger the induced currents and the opposing field. The eddy current probe envisions this construction of secondary fields for exploration of the space between the probe and the target substance.

Industrial Applications of Eddy Current

There are various industrial applications in which eddy currents are exploited, without eddy it would not function properly. Some examples are magnetic brake, electromagnetic damping based applications, induction heater, electric power-meter, electromagnetic levitation, metal characterization, vibration and position measurement, structural testing , etc. Some of them are explained in details:

  • Magnetic levitation and repulsive effect: Here eddy current based repulsive force is used in various applications. The basic parameters have been exploited across applications in magnetic levitation. This force can lift heavy objects against gravity like trains, monorails etc. This system also works independently of friction.
  • Induction furnace : Eddy current can be used for melting and welding metals, for refinishing purposes, or for manufacturing alloys. In a coil based heater, a high-freq AC is allowed to be carried through a coil which informs the metal concerned to be molten.
  • Magnetic braking  in trains: Generally, trains are moving at high speed, therefore, the braking system of trains should be effective in smooth transition free form jerking. The eddy current effect is introduced by the strong electromagnet, located directly above the rail, this rail activates the eddy current in the rail in the opposite direction of wheel rotation. It is frictionless, so there is no mechanical bonding; Thereafter, this brake operates on smooth transitions without jerking effect, but applies only to an electrically powered train.
  • Electromagnetic damping-based applications : Some gauges or instruments i.e. galvanometers use the effect of eddy current. Those non-magnetic fixed cores by metallic materials are used to generate eddy current coil oscillations, which in turn oppose the motion of the coil and cause it to come to rest by that opposing forces.
  • In adjustable speed drives:  Variable speeds can be achieved by having an eddy current coupled speed drive as required for various industrial applications.

** Although eddy current is undesirable in some applications, it can cause unwanted magnetic interference in the desired signal. Where we apply high field magnets, analysis for the error field created by the eddy, needs to be calculated and taken care of for better accuracy.

Application for eddy current sensor

Eddy Current Sensor

Eddy current sensors are extensively employed in industry due to its stability and applicability under extreme conditions. such as

  • Eddy current sensors measure vibration in steel galvanizing plant
  • Pipe or hollow-tube also used eddy current sensors to measure the thickness of a metal plate of sheet metal
  • The cylinder position movement in an internal combustion engine also has an eddy current sensor.
  • To measure the speed of hydraulic cylinders, the eddy current sensor may also be useful.
  • Work is done in aero planes like doorlocking switch and landing gearflap etc.

detection principle

High-frequency magnetic fields are used in the eddy current method. This high-freq. Sometimes a magnetic-field is created by flowing high-freq currents within a coil placed inside an eddy current sensor called a probe or sensor-head. Suppose a target (metal) is introduced into this magnetic field. In that case, electromagnetic induction causes a magnetic flux to pass over the surface of the thing. Eddy current flows vertically. This causes the eddy current sensor to change. Therefore, distance can be measured by this process.

Eddy current type displacement detectors produce a viable magnetic field by employing the high-frequency present in the detector head. When a measurement object (metal) is inside this magnetic field, surplus current is generated around the magnetic field, which passes through the surface of the object due to electromagnetic induction result. This affects the impedance of the coil inside the detector head.

As the space between the measurement item (the alloy) with the sensor tip becomes smaller, more current is generated, and the energy loss in the eddy current sensor tip increases. Because of this, once space is closer, the oscillations become smaller. As the space becomes greater, the oscillation becomes greater. The detectors correct the variant in the oscillation, which triggers a change of DC voltage. But, the linearization is fixed through linearization, and a result proportional to space can be found.

Sometimes there is mutual interference in that measurement.

parallel mounting

There are several ways to intervene; Some of them are as follows:

  • The eddy current sensor is to be installed with a spacing, so that there is no interference.
  • With a different freq type, need to be installed.
  • Given the interference prevention function need to be installed.